1. Field of the Invention
The present invention is related to the field of redundant alternating current power switching systems.
2. Background Art
Redundant electrical power distribution systems are used where electrical power must be delivered to one or more loads at all times. Redundant electrical power distribution systems allow the loads to continue consuming power during an event that disrupts a flow of power from a primary power source. Power distribution systems, where reliability or safety are extremely important, will often use double or even triple redundant connections to two or three independent power sources. Success or failure of the redundancy is dependent upon the ability to tolerate a power disrupting event in one or more of the power sources and failures within the power distribution system itself.
A variety of events may take place that may result in disruption of power to the load. A typical power disrupting event is a failure in one of the power sources feeding the redundant power distribution system. These events may occur at any time for a variety of reasons. Another common power disrupting event is human error. For example, a person intending to remove electrical power from one load may accidently switch off an incorrect circuit breaker and thus remove electrical power from the critical load instead. Power disruption may also be initiated intentionally by a person performing a maintenance action on the power distribution system. A third event is hardware failure in the power distribution system.
Existing redundant power distribution systems account for the disruption events using one of several techniques. One approach is to provide full redundant systems between independent AC power sources an a common set of DC bus bars that supply the load. Here, duplicate sets of wiring, circuit protection, safety devices and AC/DC power converters are operated simultaneously and in parallel to deliver DC electrical power to the bus bars at all times. A disruption in any one set is unnoticed by the load. This approach, however, has several disadvantages. One disadvantage is that the AC/DC converters must be co-regulated to provide one common voltage at the bus bars. Another disadvantage is that large gauge wires must be used between the bus bars and the load(s) to minimize I2R losses at the lower DC voltages. Still another disadvantage is that the load must be designed to operate from a DC voltage input, whereas most industrial and commercial loads are designed to operate from AC voltage.
Another common redundancy approach is to provide an AC switching mechanism that can connect the load to one of several AC power sources at any given moment. Advantages of this approach are that it is simple to implement and it results in AC power being directed to the load instead of DC power. A disadvantage of this approach is that a single point failure in the AC switching mechanism may result in no electrical power reaching the load. Another disadvantage is that the switch may fail in a mode that shorts two of the independent power sources together.
Where electrical power is selected from only one of several power sources at a time, the redundant power distribution system must have a mechanism for deciding which power source to use, and when to transition to another power source. A fixed master/slave approach is commonly used to make these decisions. The power distribution system defaults to receiving power from a fixed master power source. When the master power source fails, the power distribution system switches the load to the slave power source. In the event that the master power source recovers, the power switching system returns the load to the master power source. An advantage of this method is simplicity. Only the master power source must be monitored for failures, and if none are detected, then the master power source is selected to drive the load. A disadvantage is that an oscillating between the master and slave power sources may result when the master power source is experiencing an intermittent failure, or is producing marginal quality power.
What is desired is a power distribution system that allows AC electrical power to be provided from one or more independent AC power sources to an AC load. The power distribution system should have an ability to detect faults in the power sources and itself, and respond by switching the load to another power source. Transitioning between power sources should be performed only when necessary to maintain electrical power to the loads.
The present invention is a redundant power distribution system and method of operation for transferring electrical power from one of several power sources to a load. The system comprises one or more copies of a switching circuit with each switching circuit connectable to an independent power source. Each switching circuit monitors a voltage representative of its power source. The voltage being delivered to the load is also monitored by each switching circuit to detect a failed-open switching circuit. Arbitration is performed among the switching circuits detecting healthy power sources to select one power source to drive the load. Once a switching circuit has won arbitration, it continues to transfer power to the load until its power source fails or the switching circuit itself fails.
In an alternative embodiment, the switching circuit includes a self-test capability to detect a failed-closed condition of a main switch. A line switch in series with the main switch is opened upon detection of a failed-closed condition of the main switch. A safety switch may also be connected in series with the main switch and wired to electrically isolate a dangling plug should the switching circuit become unplugged from its power source.
At the system level, the load voltage sensing function of the individual switching circuits may be combined into one load sensor for the entire system. Likewise, the arbitration among the switching circuits function may be allocated to a single arbitrator external to the switching circuits. Furthermore, control of the main switches of the multiple switching circuits may be allocated to a single control function for the system. Here, arbitration among the switching circuits is selection of one main switch to power the load. Physically partitioning the circuit into two or more removable modules makes it possible to perform non-disruptive maintenance while the load continues to receive power.
Accordingly, it is an object of the present invention to provide a switching circuit, system of switching circuits, and method for controlling transfer of electrical power to a load from one power source at a time selected from among one or more power sources. In operation, a power source selected to drive the load will continues to drive the load until it fails or its associated switching circuit fails.